I love my 2009 Wrangler Unlimited but at highway speeds with the top off my seat belts flutter like a chain saw. So I came up with a quick, cheap and easy fix. Used some of my excess parts from my boat (snaps and a strap) and created these "retainers" The first thing I wanted to make sure it did NOT effect the use of the seat belt and second it would stop the fluttering. Here are pictures of the mod. They are about 8 inches above the bottom opening of the seat belt strap.

Yes greese. The bulk centers the belt so it can't flap against the plastic and the seat. Skinnier pads are better than fatter ones, since the fatter ones inhibit the sliding function of the seat belt, if you actually use it. They do make a huge difference on the front seats. On the backseat of the 2 doors I wrapped velcro around the roll bar , capturing that section of the belt with it. Loosely it too leaves the belts functional, while cutting way down on the flapping.

This is an old thread but I thought I'd respond with my simple flutter fix. If you have kids with bicycle helmets they always come with extra foam pieces that stick on the inside of the helmet to adjust size. They are about 1/2" thick with a fabric facing. I stuck one vertically, on each pillar where the seatbelt comes up out of the retraction point. This holds the seatbelt off the pillar without interfering with its operation. The seatbelt flaps against the foam but as there is nothing flat to flap against there is no flutter noise.

The cost is basically free if you're like me and never throw anything away and have some of these lying around.